Re: Concurrency - Cursor Pin:S

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: Sun, 18 Jun 2017 17:28:21 +0000
Message-ID: <LO1P123MB1026CB540A6C5D94B2BAE937A5C70_at_LO1P123MB1026.GBRP123.PROD.OUTLOOK.COM>

For an idea of the actual impact you could compare (with an interval) the number of executions of the cursor with the number of waits for the pin - multiply the average wait time by (pin waits / cursor executions). Of course you can't guarantee that all the waits are for the same cursor, but it's a ballpark figure. You've also got the elapsed time for cursor execution (from v$sql or the AWR/statspack) so you can compare that with the pin time.

Have you looked at dbms_shared_pool.markhot ? This will make Oracle generate multiple copies of a single cursor, hashing to different library cache hash buckets, and may be an easy way to bypass the pin waits.

I have some notes to write up of a recent experience of using this, but haven't got around to it yet, but the blog Mladen referenced before has an article on it: http://andreynikolaev.wordpress.com/2011/05/01/divide-and-conquer-the-true-mutex-contention/

Regards
Jonathan Lewis

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From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> on behalf of Ravi Teja Bellamkonda <raviteja.bellamkonda7_at_gmail.com> Sent: 18 June 2017 17:57:22
To: gogala.mladen_at_gmail.com
Cc: oracle-l
Subject: Re: Concurrency - Cursor Pin:S

Hi All,

Thank you for inputs. Unfortunately I cannot go ahead with the test scenario in the page as this database is an AWS RDS (which I already hate for obvious reasons) and I cannot access the x$ views.

Can someone help me provide an insight on how much the performance is being impacted as the average wait caused by cursor pin : S is 14 ms while we are in parallel trying to reduce the number of calls (App side).

Any help is very appreciated.

On Sun, Jun 18, 2017 at 9:45 AM, Mladen Gogala <gogala.mladen_at_gmail.com<mailto:gogala.mladen_at_gmail.com>> wrote: On 06/18/2017 11:52 AM, GG wrote:
W dniu 2017-06-18 o 17:41, Ravi Teja Bellamkonda pisze: Hi All,

We have a 11.2.0.4 database and recently we are facing a performance issue. Wait event is cursor pin:s. A particular sql query is executed multiple times

Executions - 31,173,028
Total Elapsed Time - 2033.64 s

Can some provide some input a way to diagnose the root issue. I see that the no of executions are very high.

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Thanks & Regards,
Ravi Teja Bellamkonda

Hi,

 start here :

https://andreynikolaev.wordpress.com/2011/04/22/cursor-pin-s-mutex-contention-testcase-and-diagnostics-tools/

Regards .

G

Nice page, however not very useful to find a medication. Pin:S is waiting for a pin in the shared mode on the cursor. I have played with another pin, "buffer busy wait", deliberately starting 10 threads from a Perl script, all modifying the same blocks in the table and performing rollback at the end. The table was SCOTT.EMP, so that the entire PK index fitted into a single block. I got my buffer busy waits all right, but the question is what to do about them? The solution I have found is to use multi-threaded version of Oracle 12c on Linux. Pins are much cheaper on the multi-threaded version then on the multi-process version. I have not yet had time to verify the reason I suspect, I am flying to the customer this afternoon and will probably not be able to verify it until mid-July, but I suspect that the multi-process pin mechanism is using semaphores, while multi-threaded does not. It will take a session with strace to verify that. I have already written about multi-threaded execution here:

https://dbwhisperer.wordpress.com/2013/10/04/multi-threaded-oracle-12c-architecture-on-linux-2/

However, I have not yet verified the details of the pin mechanism. It is my ASSUMPTION that multi-threaded version doesn't use semaphores, as opposed to the multi-process mechanism. That remains for me to do.

Regards

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Mladen Gogala
Oracle DBA
Tel: (347) 321-1217<tel:(347)%20321-1217>

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Thanks & Regards,
Ravi Teja Bellamkonda
Ph: (816)-905-7577.
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http://www.freelists.org/webpage/oracle-l Received on Sun Jun 18 2017 - 19:28:21 CEST